1. Technical Field
This invention generally relates to computer systems, and more specifically relates to computer systems that include multiple logical partitions on multiple nodes.
2. Background Art
The combination of hardware and software on a particular computer system defines a computing environment. Different hardware platforms and different operating systems thus provide different computing environments. In recent years, engineers have recognized that it is possible to provide different computing environments on the same physical computer system by logically partitioning the computer system resources to different computing environments. The series computer system developed by IBM is an example of a computer system that supports logical partitioning. If logical partitioning on an series computer system is desired, partition manager code (referred to as a “hypervisor” in IBM terminology) is installed that allows defining different computing environments on the same platform. Once the partition manager is installed, logical partitions may be created that define different computing environments. The partition manager manages the logical partitions to assure that they can share needed resources in the computer system while maintaining the separate computing environments defined by the logical partitions.
More recent developments in the field of logical partitioning support dynamic allocation of resources as logical partitions are running, and support defining logical partitions across multiple nodes in a computer system. The ability to dynamically allocate resources as logical partitions are running gives rise to issues relating to memory affinity in a logical partition. Memory affinity for a logical partition is defined herein to be the ratio of the logical partition's memory per node to the logical partition's processors per node. Memory affinity is important when logical partitions may span across nodes because an access by a processor in a logical partition in a node is much faster to memory on the same node than to memory on a different node. For this reason, we want to assure that each processor on a node has corresponding memory. If this is possible, perfect memory affinity may be achieved.
When logical partitions are first started, the partition manager typically creates the logical partitions with perfect memory affinity, meaning that each processor has a corresponding amount of memory on each node, or with similar memory affinity for all logical partitions if perfect memory affinity cannot be achieved. Note, however, that dynamic reallocation of resources may negatively impact the memory affinity of one or more logical partitions. As time goes on, it is possible for the memory affinity of a logical partition to degrade to the point of being a significant negative impact on performance of the logical partition. Without a way to dynamically adjust resource allocation to improve memory affinity in a logical partition, degradation in memory affinity due to dynamic allocation of resources may result in a logical partition that does not have the desired performance.